1. Field of the Present Invention
The present invention relates to a communication device and a channel estimation method for performing a channel estimation in a multicarrier transmission system.
2. Description of the Related Art
Over the recent years, an OFDM (Orthogonal Frequency Division Multiplexing) system has been adopted for transmission methods of a variety of communication systems, and has realized high-speed data communications at high frequency availability efficiency. The OFDM system is a system of dividing transmission data into plural pieces of data, mapping the divided pieces of transmission data to a plurality of orthogonal carrier waves (subcarriers), and transmitting the data in parallel on a frequency-axis. An OFDM modulation process involves utilizing an Inverse Discrete Fourier Transform (which will hereinafter be abbreviated to IDFT), an Inverse Fast Fourier Transform (which will hereinafter be abbreviated to IFFT), etc, while an OFDM demodulation process involves utilizing a Discrete Fourier Transform (which will hereinafter be abbreviated to DFT) and a Fast Fourier Transform (which will hereinafter be abbreviated to FFT), etc.
Further, the communication system using the OFDM system generally carries out a phase correction due to channel fluctuations and therefore conducts synchronous detection using pilot signals. The synchronous detection using the pilot signals will hereinafter be briefly described.
A transmitting device inserts the pilot signals defined as known signals into data signals, and transmits the signals that are OFDM-modulated based on the IDFT etc. A receiving device, when receiving the signals, outputs the signals of frequency components corresponding to the respective subcarriers through a process such as the DFT from the received signals, and performs the synchronous detection on the basis of the output signals. The receiving device, on the occasion of performing the synchronous detection, estimates (channel estimation) a propagation path characteristic acting on a pilot symbol contained in the output signal from the pilot symbol, and interpolates the data signals based on the acquired channel estimation values. For example, in a case where the phase fluctuation on the propagation path is interpolated, the phase fluctuation is estimated from a reception phase of the pilot signal on the basis of a modulation phase of the known pilot signal, whereby a phase fluctuation of the data signal is interpolated.
In the OFDM system, the channel estimation using the pilot signals is conducted broadly, and it is known that accuracy of the channel estimation value greatly contributes to communication performance related to a reception error rate etc. Then, a variety of techniques have been proposed for this type of channel estimation method.
The simplest channel estimation method is Zero-Forcing (which will hereinafter be abbreviated to ZF). The ZF is a technique that utilizes, directly as the channel estimation value, an estimation value of a temporary Channel Frequency Response (which will hereinafter be abbreviated to CFR) obtained from a correlation between the known pilot signal and the received pilot signal.
Further, a technique, which utilizes as the channel estimation value what the temporary CFR estimation values obtained by the ZF are further averaged on the time-axis and/or the frequency-axis, has been proposed as a technique for raising the accuracy of the channel estimation (refer to Non-Patent document 1). The Non-Patent document 1 discloses, as an OFDM-based channel estimation method, a method of increasing a Signal-to-Noise Ratio (which will hereinafter be abbreviated to SNR) of the channel estimation value by averaging the plurality of subcarriers in the frequency-direction.
Further proposed is a method of converting the temporary CFR estimation values into those in a time domain by use of the IFFT etc, then conducting a process of putting a weight to an acquired-from-the-conversion Channel Impulse Response (which will hereinafter be abbreviated to CIR) (delay profile) in the time domain and removing a noise, and thus utilizing CFR estimation values as the channel estimation values obtained by executing again the FFT (refer to Non-Patent document 2). The Non-Patent document 2 discloses a method of performing the channel estimation in a way that deletes (zero (0) replacement) the time domain with the small SNR by use of such a general property of the propagation path that received power of the CIR acquired in the way described above concentrates on a fixed area on the time-base.
Moreover, the Patent document 1 discloses a method of reducing symbol-to-symbol interference and carrier-to-carrier interference due to delayed waves over a guard interval (GI) in a receiving device based on the OFDM system.
[Patent document 1] Japanese Patent Laid-Open Publication No. 2004-208254
[Non-Patent document 1] [Characteristics of Downlink Broadband Packet TD-OFCDM in the Case of Utilizing Repetitive Channel Estimations] written by Shin, Abeta and Sawahashi, Singaku Gihou RCS2000-186, The Institute of Electronics, Information and Communication Engineers, January in 2001.
[Non-Patent document 2] Jan-Jaap van de Beek, O. Edfors, M. Sandell, [On Channel Estimation in OFDM Systems], Vehicular Technology Conference, 1995IEEE45th Volume 2, 25-28 Jul. 1995, P. 815-819
The method of raising the accuracy of the channel estimation by averaging the temporary CFR estimation values, i.e., the estimation method using an averaging in the frequency domain, has a problem that the channel estimation can not be done with the high accuracy because of being disabled to properly average if a delay dispersion is large.
Further, the channel estimation method of putting the weight on the CIR in the time domain and removing the noise etc has such a problem that since the CIR has a spread in the time domain, the process for removing the noise etc as by weighting, zero-replacement and zero-addition causes a distortion of the signal component, resulting in a decline of accuracy of the channel estimation. For example, as described above, in the method of deleting the time domain with the small SNR in the CIR, generally the received power of the CIR does not concentrate entirely on a fixed time domain but has a spread to some extent in the time domain with the result that the spread signal components might be removed together with the noise components, and therefore a problem is that it is impossible to perform the channel estimation with the high accuracy in the time domain with the high SNR.